What is G-D? is that God? if so why did you write it that way?
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HosseinFeb 6 '13 at 9:49

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@Hossein, welcome to Mi Yodeya! As you may notice throughout the site, spelling it that way is a common convention among many religious Jews, although is seems from here that it may not be required in Jewish law. Take it from me, though, as an English-speaking Jew, it is very common.
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Seth JFeb 6 '13 at 14:52

@Sethj:Thank you very much :) it was very informative and interesting ;)
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HosseinFeb 7 '13 at 11:00

As HodofHod mentioned, once a person converts to Judaism, mentally and bodily, the conversion is full and effective spiritually and legally (meaning within Jewish law).

Aside from a few cases of very strict rabbinic interpretations of what is considered a "full and effective" conversion (and attempts, thereby, to retroactively invalidate a person's conversion), and different standards among different denominations, nobody disputes that once someone has effectively converted they remain Jewish, in the eyes of Jewish law, forever.

Just to add to that answer and clarify/expand a bit, there is no specific sin against "reverting", because no such thing is recognized. However, any time a convert engages in any act that is forbidden to Jews, the convert is sinning as a Jew by not upholding the commandments in the Torah - not because the convert has "reverted".

@saber tabatabaee Yazdi that isn't necessary, as my answer was just an enhancement. I would have edited Hod's answer if I weren't concerned about making too drastic of a change.
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Seth JFeb 6 '13 at 13:27

@Saber You should accept whichever answer is most helpful to you. :)
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HodofHodFeb 6 '13 at 15:43

Three details that weren't addressed by the other (well-written) answers:

1) The Talmud in Kesubos 11a says that a child who converts has the ability to turn back at the time of their coming of age

אמר רב יוסף הגדילו יכולין למחות

When they grow up, they may protest [their conversion].

This is age 13 for a boy, and 12 for a girl, although see the Tosefos HaRi"D there who says that it is even earlier than that. The Chasam Sofer also entertains that it would be an earlier age.

Such a convert may change their mind on their conversion at that point.

Many commentaries assume this to mean that the conversion of a child is not a done deal until such point, not that they are a full-fledged Jew who can change their mind. However, Tosefos in Sanhedrin 68b s.v. קטן אי אתה צריך לחזור implies that he/she is a full-fledged Jew who becomes not a Jew at the point of their refusal.

2) If a non-Jew converts without having been properly vetted by a valid Jewish court to determine his/her sincerity, there are opinions that say that even though we do not disregard their conversion, we treat them as a questionable convert until their pure intentions have been made clear (see Responsa Achiezer 3:26:3 - חיישינן לחומרא עד שתתברר צדקתו), as the validity of their acceptance of Mitzvos is questionable (I assume this means that we would treat them as a non-Jew as far as counting them for Minyan or marrying a Jew and the like). Your question gave the case of "tomorrow they return back" - the very next day. That would presumably be sufficient reason to doubt the sincerity of their acceptance, and if they had not been vetted by a Rabbinical court, it would cast doubt upon the validity of their conversion.

3) As far as the source of the statements of the other answers, it is the Talmud in Yevamos 47b:

Once he immerses and comes up, he is a Yisrael for all purposes. What Halacha does this come to tell us? That if he reverts [to his old ways] and then marries a Jewess, he is a rebellious Jew, and the marriage is binding.